Polymer-nanocrystal photo device and method for making the same

ABSTRACT

Disclosed is a method of producing a photo device containing a layer of nanometer sized particles and a conducting polymer in solid sate, wherein the layer is made by mixing the nanometer sized particles in colloid with precursor polyparaphenylenevinylene or a derivative thereof and wherein the nanometer sized particle is selected from the group consisting of TiO 2 , ZnO, ZrO 2  and SnO 2  and the layer is heated to a temperature during a period of time.

Especially for reducing costs for photovoltaic cells, research is done all around the world for finding a solid state composition of creating an interpenetrating solid-state conducting material in a nanoporous network. Such device could also be used for LED (Light Emitting Diodes), photo sensors, optical switches and even optical networks.

The present invention relates to a photo device comprising a layer of nanometer sized particles and a conducting polymer in solid state, wherein the nanometer sized particles are chosen from the group of TiO₂, ZnO, CdSe, CdS, ZrO₂ and SnO₂, and wherein the conducting polymer comprises PPV (polyparaphenylenevinylene) or a derivative thereof.

Prior methods have been published on the insertion of a polymer electrolyte into a preformed nanocrystalline TiO₂ (titanium dioxide) network (F. Cao et al., Proceedings of Nanostructured Materials in Electrochemistry, held: Reno, Nev., USA, 21-26 May 1995). These methods have, however, the disadvantage of being based on an ionic conductor. The device performance is therefore limited. The ionic conductor is not suited for electronic devices.

Another prior method using conducting polymers (PPV and derivatives) replaces the inorganic nanocrystalline network with C₆₀ and derivatives which act as electron-acceptors (G. Yu et al., Science 270 (1995) 1789). However, in this method, the electron transport is restricted. Further, C₆₀ is not a material which is produced abundantly, and is expensive; it is also much less stable than inorganic materials.

Another prior method using a conjugated polymer, poly(2-methoxy, 5-(2′-ethyl)-hexyloxy-p-phenylenevinylene; MEH-PPV), incorporates nanoparticles of CdS or CdSe (N. C. Greenham et al., Physical Review B. Condensed Matter, 54 (1996) 17628, N. C. Greenham et al., Synthetic Metals 84 (1997) 545-546), but the cadmium-containing compounds have a disadvantage in that they are carcinogenic and also not abundantly available.

Another publication (S. A. Carter et al., Applied Physics Letters 71 (1997) 1145) discloses the use of TiO₂ nano-particles blended with electroluminescent organic material in light emitting diodes. As an example, MEH-PPV was tested as organic material. The mixtures were spin cast onto an Ito coated glass, whereafter the solvent was evaporated off. The device lacks photovoltaic properties.

The present invention has for its object to offer a solution to the problem of creating an interpenetrating solid-state conducting material in a nanoporous network, for the purpose of creating stable, all-solid state photovoltaic cells. The instability problems associated with liquid based electrolytes in nanoporous networks (B. O'Regan et al., Nature, 353 (1991) p737) are to be avoided.

A further object of the present invention is to offer a photovoltaic cell, which offers the stability, electron transporting properties, the abundant availability, the low costs, and the no-toxicity of TiO₂ in an interconnected network in combination with the hole-transporting, light-absorbing, stable properties of PPV (poly-paraphenylenevinylene).

A further aspect of the present invention is the ease of manufacturing, namely in a single thermal treatment of the inorganic and organic materials together. The thermal treatment is elegantly simple, and provides for converting the polymer precursor to its final, conducting form as well, as for producing electrical contact between the inorganic nanometer sized particles to make continuous electron-carrying pathways to the end terminal of the photovoltaic cell, the object of the present invention.

None of the above prior art methods use a mixture of a polymer precursor in solution and inorganic nanometer sized particles in a colloidal solution to produce an interpenetrating composite layer in a single step of thermal treatment.

A preferred embodiment of the method according to the present invention uses a conducting polymer precursor (poly(p-xylene-alpha-tetrahydrothiophene-bromide; p-PPV) and a nanocrystalline material, TiO₂.

Another preferred embodiment of the method of the present invention is that the mixture of PPV precursor or derivative there of and TiO₂ is between 10:90 and 70:30 by weight, preferably approximately 50:50.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and details of the present invention are elucidated in the following description, which refers to the enclosed figures, in which:

FIG. 1 is a diagrammatic cross section view of a device according to the present invention; and

FIG. 2 is a graph of current voltage characteristic of the device of FIG. 1.

A schematic cross section of the cell is shown in FIG. 1. The polymer p-PPV was made by chemical synthesis (J. J. M. Halls et al., Nature 376 (1995) 498), the TiO₂ nanocrystals were obtained from Degussa AG Corporation, Germany. An approximately 0.7% methanolic solution of the p-PPV was combined with a colloid of TiO₂ (the colloid had been made according to Nazeeruddin et al., J. Am. Chem. Soc. 115 (1993) 6382), to give a mixture of approximately 1:1 p-PPV and TiO₂ by weight. This mixture was spin-coated on a glass substrate 1 with a transparent conducting coating 2, in this example SnO₂:F, to give a thin film 3. The film was heated to 320° C. for ten hours in vacuum and an aluminum contact 4 was evaporated in such a way that there was no overlap with the transparent conducting coating on the glass on top of the film to complete the cell.

Current-voltage (IV) curves of the cells in the dark and in substantially white light were measured at terminal 5 and 6 (FIG. 1) and a plot of this data is shown in FIG. 2 which demonstrates that the cells produce electric power under illumination.

At least the thermal treatment step of the present invention is novel and inventive over the prior art. This step provides for both conversion of the p-PPV to its conducting form, PPV and for producing electrical contact between the TiO₂ particles to produce electron-carrying paths through the film.

The method according to the present invention allows the use of precursor polymer and thus does not require the complex process of first synthesizing MEH-PPV before blending with nanoparticles. Moreover, the method, by starting from a mixture of a colloid of TiO₂ and a solution of p-PPV, also provides a solution to the very poor penetration of conducting polymers with high molecular mass into a preformed porous nanocrystalline semiconductor.

The present method is first in making operational photovoltaic cells of a conducting polymer, such as PPV, and a sintered electrically interconnected network of nanocrystalline particles, such as TiO₂.

PPV has been shown to be a good hole-conducting material and TiO₂ an excellent electron transporter, so their combination together, prepared in a single thermal treatment step, provides a significant advantage.

The present invention is not limited to the embodiment described; the rights applied for are defined in the annexed set of claims; the scope of protection includes other polymers and nanocrystals, as well as other temperatures. 

What is claimed is:
 1. A method of producing a photo device, comprising a layer of nanometer sized particles and a conducting polymer in solid state, characterized in that the layer is made by mixing the nanometer sized particles in a colloid with precursor PPV (polyparaphenylene-vinylene) or a derivative thereof, the nanometer sized particles being selected from the group consisting of TiO₂, ZnO, ZrO₂, SnO₂ and mixtures thereof, wherein the layer is heated to a temperature during a period of time.
 2. A method according to claim 1, wherein the precursor PPV or derivative thereof is poly(p-xylene-alpha-tetrahydrothiophene bromide) and the nanometer sized particles consist of TiO₂.
 3. A method according to claim 2, wherein the mixture of TiO₂ and poly(p-xylene-alpha-tetrahydrothiophene bromide) is between 10:90 and 70:30 by weight.
 4. A method according to claim 1 wherein the heating takes place under pressure of an inert gas or under vacuum.
 5. A photo device produced by the method of claim 1, wherein the layer is a thin film of a photo voltaic cell.
 6. A method according to claim 2, wherein the mixture of TiO₂ and poly(p-xylene-alpha-tetrahydrothiophene bromide) is about 50:50 by weight. 